Radon Risk in California

California's statewide average radon level is 2.48 pCi/L, which falls below the EPA's action level of 4 pCi/L. That's a reasonable baseline, but statewide averages can mask significant county-to-county variation. Some parts of California run considerably higher than others.

Looking at the zone breakdown: 2 of California's 58 counties are EPA Zone 1 (high risk), 29 are Zone 2 (moderate), and 27 are Zone 3 (low). That means 3% of the state falls into the highest risk category, counties where the EPA predicts average indoor levels above 4 pCi/L. Use the county list below to find your area and see the most specific data available.

Within California, the spread is notable. Mono County has the highest measured average in the state at 7.8 pCi/L. Del Norte County sits at the other end with 0.1 pCi/L. 10 counties have a measured average above 4 pCi/L. Remember: these are averages. Individual homes can test significantly higher or lower than their county average depending on construction, foundation type, and ventilation.

Tipper's recommendation for California residents: if you haven't tested your home, start there. A short-term test kit costs around $15 and gives you a result in 48 hours. If your home tests at or above 4 pCi/L, a professional mitigation system (typically $800–$2,000 depending on your foundation and home size) will reliably bring levels down.